Frequently, excessive voltage or current is applied across service lines that deliver power to residences and commercial and institutional facilities. Such excess voltage or current spikes (transient overvoltages and surge currents) may result from lightning strikes, for example. The above events may be of particular concern in telecommunications distribution centers, hospitals and other facilities where equipment damage caused by overvoltages and/or current surges and resulting down time may be very costly.
Typically, sensitive electronic equipment may be protected against transient overvoltages and surge currents using Surge Protective Devices (SPDs). For example, brief reference is made to FIG. 1, which is a system including conventional overvoltage and surge protection. An overvoltage protection device 12 may be installed at a power input of equipment to be protected 50, which is typically protected against overcurrents when it fails. Typical failure mode of an SPD is a short circuit. The overcurrent protection typically employed is a combination of an internal thermal disconnector to protect the device from overheating due to increased leakage currents and an external fuse to protect the device from higher fault currents. Different SPD technologies may avoid the use of the internal thermal disconnector because, in the event of failure, they change their operation mode to a low ohmic resistance. In this manner, the device can withstand significant short circuit currents. In this regard, there may be no operational need for an internal thermal disconnector. Further to the above, some embodiments that exhibit even higher short circuit withstand capabilities may also be protected only by the main circuit breaker of the installation without the need for a dedicated branch fuse.
Brief reference is now made to FIG. 2, which is a block diagram of a system including conventional surge protection. As illustrated, a three phase line may be connected to and supply electrical energy to one or more transformers 66, which may in turn supply three phase electrical power to a main circuit breaker 68. The three phase electrical power may be provided to one or more distribution panels 62. As illustrated, the three voltage lines of the three phase electrical power may designated as L1, L2 and L3 and a neutral line may be designated as N. In some embodiments, the neutral line N may be conductively coupled to an earth ground.
Some embodiments include surge protective devices (SPDs) 15. As illustrated, each of the SPDs 15 may be connected between respective ones of L1, L2 and L3, and neutral (N). The SPD 15 may protect other equipment in the installation such as the distribution panel among others. In addition, the SPDs may be used to protect all equipment in case of prolonged overvoltages. However, such a condition may force the SPD to conduct a limited current for a prolonged period of time, which may result in the overheating of the SPD and possibly its failure (depending on the energy withstand capabilities the SPD can absorb and the level and duration of the overvoltage condition). A typical operating voltage of an SPD 15 in the present example may be about 400V (for 690V L-L systems). In this regard, the SPDs 15 will each perform as an insulator and thus not conduct current during normal operating conditions. In some embodiments, the operating voltage of the SPD's 15 is sufficiently higher than the normal line-to-neutral voltage to ensure that the SPD 15 will continue to perform as an insulator even in cases in which the system voltage increases due to overvoltage conditions that might arise as a result of a loss of neutral or other power system issues.
In the event of a surge current in, for example, L1, protection of power system load devices may necessitate providing a current path to ground for the excess current of the surge current. The surge current may generate a transient overvoltage between L1 and N. Since the transient overvoltage significantly exceeds that operating voltage of SPD 15, the SPD 15 will become conductive, allowing the excess current to flow from L1 through SPD 15 to the neutral N. Once the surge current has been conducted to N, the overvoltage condition ends and the SPD 15 may become non-conducting again. However, in some cases, one or more SPD's 15 may begin to allow a leakage current to be conducted even at voltages that are lower that the operating voltage of the SPD's 15. Such conditions may occur in the case of an SPD deteriorating.
As provided above, devices for protecting equipment from excess voltage or current spikes (transient overvoltages and surge currents) may include including varistors (for example, metal oxide varistors (MOVs) and/or silicon carbide varistors).